Superconductivity was first discovered from investigation of mercury (Hg) by H. Kamerlingh-Onnes in 1908. When a metal or alloy is cooled down below a certain temperature which is called its superconducting transition temperature (Tc), superconductivity is caused by extinction of electrical resistance. Such a superconducting phenomenon is not limited to a specific metal, but can be found in most of metals at a sufficiently low temperature, depending on the metal. For example, whereas the critical superconducting temperature of mercury is 4K, that of a certain alloy of tin and niobium is 18K.
As for superconducting material showing such a phenomenon, metal compounds were mainly used before Bednorz and Muller found superconductivity in La.sub.2-x Ba.sub.x CuO.sub.4 in 1986. Since then, copper oxides have been mainly developed for superconductors having high Tc. After that, many kinds of copper-based superconductors have been synthesized at a higher critical temperature than the boiling point of liquid nitrogen (77K). This became a turning point in studying the application of superconductors. The representative examples are YBa.sub.2 Cu.sub.3 O.sub.7, Bi.sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+4 and Tl.sub.2 Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+4.
Recently, the mercury-based superconductor HgBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8 was developed by Schilling et al in 1993, which has the highest Tc of 133K among the existing superconductors. However, it is difficult to synthesize the mercury-based compounds due to evaporation of mercury during the preparing process. Furthermore, HgBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8 is very unstable in the air, and has low critical current density so that study of its application is limited. On the contrary, thallium-based TlBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8+.delta., which has similar structure to the mercury-based superconductors is relatively stable in air. However, it has a Tc of 115K which is much lower Tc than mercury-based superconductors.